This invention relates to indicating apparatus and particularly to electrically operated indicating apparatus responsive to changes in electric current.
The use of electrically responsive indicating apparatus to measure certain physical conditions, such as temperature, pressure and liquid level is well known. Further, many of the indicating apparatus have employed devices which measure the change in the particular physical state as a function of electrical resistance, with the results of the measurement appropriately displayed on an ammeter. Indicator assemblies employing the measurement of resistance are found in U.S. Pat. No. 1,285,145 to Harrington et al., U.S. Pat. No. 1,705,158 to McCoy, U.S. Pat. No. 2,004,421 to Smulski, U.S. Pat. No. 2,333,406 to Ballard, U.S. Pat. No. 2,339,021 to Lingel, U.S. Pat. No. 2,883,623 to Hastings et al., U.S. Pat. No. 2,985,019 to Colvin, U.S. Pat. Nos. 3,013,233 and 3,013,234 to Bourns, U.S. Pat. No. 3,247,716 to Ranke, U.S. Pat. No. 3,630,087 to Ogden et al., U.S. Pat. No. 3,638,481 to Wilner, and U.S. Pat. No. 3,640,134 to Hop.
The majority of the above-listed patents are generally related to the measurement of a physical state as a function of resistance. Certain of the patents, including the patents to Bourns, Ranke and Colvin, attempt to measure differences in pressure to arrive at a pressure ratio by the employment of paired resistance elements or transducers appropriately linked. Ogden et al. and Hop attempt to balance separate pressure readings to arrive at a mean value.
The prior art represented above also disclosed a wide variety of transducer elements, ranging from the coiled structures known as Bourdon tubes, and including bellows-type resistors, piezoresistive elements, as well as simple mechanical linkage between a float member and a rheostat arm. All of the indicator assemblies discussed above possess certain deficiencies in that they are in most instances complicated devices that are costly to manufacture and maintain, or in the alternative are simple devices less expensive to manufacture however inaccurate in performance.
A need has long existed for an accurate indicator assembly to monitor, for example, a level of liquid fuel in a container such as a fuel tank, steam boiler or the like, which is subject to motion in use. Specifically, in the instance of marine vessels and aircraft, the fuel tanks are necessarily subjected to constant changes in attitude from the horizontal which renders the accurate metering of the fuel level extremely difficult. Attempts to adapt the transducers shown in the prior art have resulted in complicated, cumbersome assemblies which were generally unreliable and costly. Thus, for example, the device disclosed in the patent to Hop comprises a particularly complex assembly attempting to derive accurate measurement through a measurement of pressure differentials at various points in the fluid container, which pressure differentials are then fed into a summing and dividing circuit which is designed to arrive at a mean value. A need thus exists for a simple, space-saving and accurate indicator assembly.
In our copending application Ser. No. 824,046, the disclosure of which is incorporated herein by reference, an indicator assembly capable of attitude correction is disclosed which employs a pair of weight-responsive variable resistors disposed in fluid connection at the opposite ends of the bottom of a fluid container, and electrically connected to each other in series, to provide a liquid measurement in terms of a total resistance output which will be constant for a given volume of liquid. The device thus described is simple in construction and operation and is easily employed by direct series connection to an electric current-responsive meter, such as an ammeter or the like. Liquid level measurement is accomplished by the use of weight-responsive devices such as Bourdon tubes and aneroid bellows, which are mechanically linked to identical rheostats.